The invention concerns a digital burette having a manual drive for suctioning and delivering an adjustable dose volume, a downstream gear and a digital display device controlled thereby for setting the dose volume, wherein the control comprises an incremental encoder which is operatively connected to the gear, at least one sensor for detecting the signals produced by the incremental encoder and a processor, connected to the sensor, for calculating the dose volume corresponding to the number of signals, the processor being connected to the digital display device. The invention also concerns a method for displaying the dose volume in said digital burette.
Digital burettes are used for a plurality of volumetrical determinations, such as titration, for exact dosing of defined liquid volumes. Of particular interest is the possibility to pre-set and reproduce the dose volume to be delivered which a digital display device, e.g. a display, indicates.
The operation of such digital burettes is based on a lifting piston guided in a pipetting channel for suctioning the liquid to be dosed, wherein the lifting piston usually communicates with the liquid via an air cushion. The lifting piston is manually driven, e.g. actuated by a turning handle, wherein the rotation is transmitted via a mechanical gear to the lifting piston for suctioning or discharging the desired liquid volume. An incremental encoder is operatively connected to the gear and a sensor detects the signals generated by the incremental encoder to detect the dose volume corresponding to the number of turns of the turning handle. The sensor is connected to a processor which calculates the dose volume from the signals, displays same on a digital display device e.g. a liquid crystal display, light diodes or the like.
The incremental encoder may be a slotted disc with slots provided, one behind the other, in the peripheral direction. An optical sensor, e.g. a light barrier, detects the number of turns of the slotted disc by detecting the number of slots passing the sensor. To recognize the direction of rotation of the slotted disc and therefore the lifting direction of the lifting piston (suction or pressure stroke), a second optical sensor, e.g. a second light barrier or a forked light barrier may be provided with which the direction of rotation of the slotted disc is determined from the order of the signals produced by the two sensors.
DE 38 18 531 A1 discloses a piston burette having an incremental encoder in the form of a slotted disc cooperating with a forked light barrier to determine the rotational direction of the slotted disc and the stroke direction of the piston. EP 0 559 223 A1 discloses a bottle filling device having a similar control.
The incremental encoders may also be sector discs with two groups of sectors of different magnetic field strength, wherein the sectors of the two groups are alternately disposed in the peripheral direction of the sector disc. The sectors may be formed e.g. by permanent magnets which induce alternately different and/or opposite magnetic fields. Alternatively, only every second sector of the sector disc may comprise a magnet with the sectors disposed between the magnets being non-magnetic. A sensor is provided which is sensitive to the magnetic field and functions like a magnetic switch to open or close exclusively in response to the magnetic field induced by the respective magnet for detecting the number of turns of the sector disc. The current consumption of these sensors is advantageously small since they require only a voltage which is sufficient to detect the opening or closing position while the opening and closing process per se is effected purely mechanically due to the force field induced by the magnet of the sector disc. The sensor produces a substantially rectangular voltage or current signal when the sector disc is turned.
Digital burettes consume a relatively large amount of current since the digital display device and the overall control including sensors and, in particular, processor require current. The processor requires the greatest amount of current in dependence on its clock frequency. Replaceable dry batteries or rechargeable accumulators are used as power supplies. The non-productive times during replacement or recharging thereof are disadvantageous. They also fail prematurely under the often-corrosive conditions in the laboratory and can damage the burette, in particular when electrolyte is discharged. Digital burettes with lithium cells are also known which are soldered onto a board together with the electric and/or electronic components. Lithium cells of this type are relatively robust but the regular replacement of the cells requires dismantling or replacement of the entire board and is therefore demanding and expensive. Down times are created since long-term storage of the boards would impair the lithium cells.
It is the underlying object of the invention to overcome these disadvantages.